Receiver arrangement with AC coupling

ABSTRACT

A receiver arrangement with AC coupling is specified in which a filter arrangement ( 3 ) is provided in a baseband signal processing chain in a homodyne receiver and can be switched between at least two high-pass filter cut-off frequencies. In this case, a brief changeover is made to a higher cut-off frequency when varying the gain of a low-noise baseband amplifier ( 2 ), for example when the received field strength changes, during the reception mode. The described arrangement allows changes to be carried out to the gain in baseband during the normal reception mode. The present receiver is accordingly suitable for code division multiple access methods, such as those which are provided in the UMTS Standard.

FIELD OF THE INVENTION

The present invention relates to a receiver arrangement with ACcoupling.

BACKGROUND OF THE INVENTION

A distinction is drawn between homodyne and heterodyne architectures formobile radio receivers. While, in the case of heterodyne mobile radioreceivers, a received radio-frequency signal is first of all convertedto an intermediate frequency in order subsequently to be converted tobaseband, homodyne mobile radio receivers convert the radio-frequencysignal to baseband in only one conversion step. Homodyne mobile radioreceivers such as these are also referred to as zero-IF or as directconversion (DC) receivers and are used, for example, in the so-calledthird-generation mobile radio standard, Universal MobileTelecommunications System, UMTS.

One system-dependent disadvantage of direct conversion is DC voltageoffsets which, on the one hand, may be of a static nature, and on theother hand may be of a dynamic nature. The static offsets are causedinter alia by circuitry-dependent offsets in the individual blocks ofthe receiver chains, for example as a result of large pairing tolerancesof the components.

If the received signal is weak, that is to say the described DC offsetsmay be many times higher than the actual useful signal, the offsets arealso amplified with the baseband amplification that is required for theuseful signal, so that a signal which is too large for digitizationwould be produced at the output of the analog baseband chain and at theinput of the analog/digital converter that is normally provided there.It is therefore essential to use circuitry measures to compensate forsuch DC voltage offsets.

Conventional methods to compensate for a DC voltage offset in the analogbaseband chain are based either on the high-pass filter principle andare provided by means of simple AC couplings, or feedback loops areprovided, with the feedback path having low-pass filter characteristics.

Overall, these methods have the disadvantage that, on the one hand, alow cut-off frequency is required for the high-pass filter in order toavoid excessively distorting the useful signal, while, on the otherhand, a high cut-off frequency is required in order to ensure that thestabilization time of the AC coupling is not too long. Furthermore, manydirect converters have adaptive gain control which is dependent on thereceived field strength of the radio-frequency signal. However, gaincontrol systems such as these result in sudden changes in the gain,resulting in transient equalization processes which can exceed theuseful signal by many times, so that the analog/digital converter cannotbe driven ideally.

The described problems relating to transient equalization processesresulting from changes to gain factors are exacerbated by the fact thatsuch transients would be amplified many times further by subsequentamplifier stages, for example programmable amplifiers.

In the case of mobile radio methods such as GSM, which operate usingTime-Division Multiple access TDMA and accordingly transmit and receivein time slots, the described problems can be avoided by making changesto the gain only between time slots. In the case of mobile radio methodsfor which continuous reception is required, for example in the case ofsystems which operate using CDMA, Code Division Multiple Access methods,it is however, desirable to match the gain to the received fieldstrength even when the receiver is being operated without any pauses.

The object of the present invention is to specify a receiver arrangementwith AC coupling in which it is possible to match the gain in thereceiver during a normal reception mode.

SUMMARY OF THE INVENTION

According to the invention, the object is achieved by a receiverarrangement with AC coupling, comprising

-   -   an input for supplying a radio-frequency signal,    -   a frequency converter which is coupled to the input and produces        a baseband signal at its output,    -   a baseband amplifier with variable gain, which is connected to        the output of the frequency converter and has a control input        for varying the gain,    -   a filter arrangement for AC coupling, with an input which is        connected to an output of the baseband amplifier and with a        high-pass filter for filtering the baseband signal, with a        cut-off frequency which can be switched between at least two        values, with the lower of the at least two cut-off frequencies        being used in a normal mode and with the higher of the at least        two cut-off frequencies being used in a changeover mode, and        with a control input for varying the cut-off frequency, and    -   a control circuit for activation of the higher cut-off frequency        during the changeover mode on the basis of a readjustment of the        gain of the baseband amplifier, with an output which is        connected to the control inputs of the baseband amplifier and of        the filter arrangement.

The high-pass filter which is provided for AC coupling in the basebandsection of the signal processing chain allows the received useful signalto be transmitted with a high degree of accuracy with a lower cut-offfrequency which is set during normal operation, for example of 2 KHz. Ifthe gain ratio of the amplifier is changed, for example if the receivedfield strength changes, the process is in contrast switched to a highercut-off frequency of, for example, >1 MHz, thus allowing the filter tostabilize more quickly. The higher cut-off frequency is in this caseactivated for a time interval which can be defined, for example of a fewmicroseconds, and the process then switches back again to the lowercut-off frequency, that is to a longer time constant.

The described capability to switch the cut-off frequency for the ACcoupling thus allows matching or slaving of the amplifier power to thereception conditions for example of a mobile station, so that thereceiver present is suitable for mobile radio methods which do notoperate with time slots but in which continuous reception operation mustbe ensured.

The described change to the gain of the baseband amplifier, which ispreferably designed as a low-noise amplifier, with variable gain ispreferably carried out using so-called soft switching, that is to saywithout any abrupt, sudden transition.

Since the described arrangement makes it possible to drive ananalog/digital converter that is connected downstream from the receiverarrangement at an optimum operating point, this analog/digital convertercan be constructed to be less complex and with less resolution.

The described arrangement allows changes to the gain during normalreception.

In one preferred embodiment of the present invention, the controlcircuit is designed such that the higher cut-off frequency is activatedfor a variable time interval, which starts at the time of the change tothe gain of the baseband amplifier. After a change to the baseband gain,the process is switched to the higher cut-off frequency for a fewmicroseconds, for example, in order to allow rapid stabilization and atransient equalization process of short duration.

In a further preferred embodiment of the invention, the higher cut-offfrequency is greater than or equal to 1 MHz.

In a further preferred embodiment of the present invention, the filterarrangement comprises a low-pass filter which can be connected and iscoupled to the control circuit in order to activate the low-pass filtereffect during the changeover mode.

The low-pass filter which can be connected and is preferably connectedupstream of the high-pass filter avoids a sampling effect when thehigher cut-off frequency is activated, as can occur in particular whenthe useful signal is not significantly less than the sudden DC voltagechange that is caused by amplifier switching. A baseband signal that hasbeen subjected to low-pass filtering is in this case supplied to thehigh-pass filter with the higher cut-off frequency during the activationof the higher cut-off frequency. This low-pass filtering effect reducesthe sampling effect in particular in the case of large adjacentchannels, that is to say when the filter arrangement is connectedupstream of the channel filter in baseband. However, the described ACcoupling with the filter arrangement is in fact required at the start ofthe baseband chain in order to eliminate DC offsets which occur as aresult of the frequency conversion and are caused, for example, bymismatches in the mixer.

In a further preferred embodiment of the present invention, aprogrammable amplifier is provided, is coupled to the output of thefilter and is connected to the control circuit, in order to set thelowest gain which can be set during the changeover mode.

With the described activation of the smallest gain which can be set forthe programmable amplifier, the suppression of a transient that iscaused by switching of the gain factor can be reduced further. Theduration of such blanking of the programmable amplifier in this casedepends on the stabilization time of a channel filter which is providedin the baseband section, and preferably corresponds to the time durationof the activation of the higher cut-off frequency in the filterarrangement by means of the control circuit. By way of example, this maylast for a few microseconds.

As an alternative to the described blanking of the programmableamplifier, further means for AC coupling may be inserted into thebaseband chain. The described setting of the smallest gain which can beset for the programmable amplifier avoids any additional complexity,however, since the programmable amplifier is normally provided in anycase and invariably has a control input for setting or programming thegain ratio.

In a further preferred embodiment of the present invention, a basebandfilter is provided, and is connected between the filter arrangement andthe programmable amplifier. The baseband filter is used in particularfor channel filtering, that is to say for suppression of undesirableadjacent channels.

In a further preferred embodiment of the present invention, the receiverarrangement is designed for processing balanced signals.

If a local oscillator signal for down-mixing the radio frequency to thebaseband frequency supplied to the described frequency converter is onthe one hand left unchanged and is on the other hand supplied to thedescribed frequency converter with a phase shift of 90°, and if thebaseband chain is thus designed for transmitting complex-value signals,then both the in-phase signal path and the quadrature signal path caneach be designed as balanced signal paths. In this case, the describedbaseband chain, with a frequency converter, a low-noise basebandamplifier with an adjustable gain ratio, a filter arrangement, abaseband channel filter and a programmable amplifier, is provided inboth the I signal branch and in the Q signal branch.

In a further preferred embodiment of the present invention, thehigh-pass filter in the filter arrangement comprises a parallel branchin a balanced signal path, which has a resistor with aparallel-connected first switch. Furthermore, a series branch is in eachcase connected upstream of the parallel branch in the two lines whichare designed for the transmission of balanced signals, and each have acapacitance.

The high-pass filter, which is constructed in analog form with resistorsand capacitances, can thus be switched in a simple manner by means ofthe described first switch between two time constants and thus betweentwo lower cut-off frequencies for the high-pass filtering. Instead ofthat described, other analog circuits may also be provided in order toform a high-pass filter.

The first switch is in this case opened during normal operation, so thatthe resistance in the parallel branch is effective, accordinglyactivating a long time constant of, for example 2 KHz. In the changeovermode, the first switch is closed, thus resulting in a high cut-offfrequency of, for example, 1 MHz.

In a further preferred embodiment of the present invention, the seriesbranches of the balanced signal path each comprise a parallel circuitformed by a resistor and a second switch, and these are in each caseconnected upstream of the described capacitances in the series branches.

The second switches are closed during normal operation, so that theseries resistances are bridged by a short circuit. In a changeover mode,on the other hand, the second switches are opened, so that thecapacitance C across the series resistance follows signal changes at theinput, thus achieving low-pass filtering. This low-pass filtering avoidsany sampling effect that may occur. Instead of that described, someother analogously constructed low-pass filtering may also be provided,and is connected upstream of the high-pass filter with the switchablecut-off frequency.

In a further preferred embodiment of the present invention, the controlcircuit is designed such that in order to activate the changeover mode,the second switches are first of all opened and the first switches arethen closed, and in that, in order to return to the normal mode, thefirst switches are first of all opened, and the second switches are thenclosed.

The described switching sequence is necessary in order to reliably avoidthe sampling effect. Accordingly, the low-pass filtering is activatedbefore the high-pass filter is switched to the higher cut-off frequencyand, when switching back to normal operation, is first of all switchedback to the low cut-off frequency for high-pass filtering, only afterwhich is the low-pass filtering cancelled.

Further details of the invention are the subject matter of the dependentclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in more detail in the following textusing a number of exemplary embodiments and with reference to thedrawings, in which:

FIG. 1 shows a first exemplary embodiment of the high-pass filter forthe filter arrangement.

FIG. 2 shows the reduction in the equalization processes which can beachieved with the switchable cut-off frequency of the arrangement shownin FIG. 1 when changing the gain ratio.

FIG. 3 shows the transient response of a baseband signal processingchain with a filter arrangement as shown in FIG. 1, as well as adownstream channel filter and a programmable amplifier on the basis of astep-function response at the output of the programmable amplifier,

FIG. 4 uses a signal profile at the baseband output to show the samplingeffect which occurs in particular when the useful signal is notsignificantly smaller than the sudden DC change which occurs byswitching the gain at the start of the baseband chain.

FIG. 5 uses an example to show a development of the filter arrangementshown in FIG. 1 with additional low-pass filtering in order to avoid asampling effect,

FIG. 6 shows the signal profile as shown in FIG. 5 by analogy to FIG. 4,

FIG. 7 uses a block diagram to show the receiver arrangement with afilter arrangement for example as shown in FIG. 1 or 5, and

FIG. 8 shows the signal profiles of the control signals of the basebandamplifier, filter arrangement and programmable amplifier as shown in theblock diagram in FIG. 7.

Reference symbols in the drawings are:

-   1 Frequency converter-   2 Low-noise baseband amplifier-   3 AC coupling-   4 Channel filter-   5 PGC-   6 Control circuit-   A Transient-   B Step-function response-   C Sampling effect-   D Signal profile-   E Control signal-   F Control signal-   G Control signal-   C1 Capacitor-   C2 Capacitor-   R1 Resistor-   R2 Resistor-   R3 Resistor-   R4 Resistor-   S1 Switch-   S2 Switch-   S3 Switch-   S4 Switch-   t₀ Start of the changeover mode-   ΔT Changeover mode interval

DETAILED DESCRIPTION

FIG. 1 shows a filter arrangement for AC coupling for use in a balancedreceiver, to be precise in its baseband signal processing chain andpreferably downstream from a low-noise variable-gain baseband amplifierwhich is arranged at the signal output of a frequency converter fordirect conversion.

The arrangement shown in FIG. 1 has a respective series capacitance C1,C2 in the two series branches of the balanced baseband signal path. Onthe output side a respective resistor R1, R2 is connected to thecapacitances, C1, C2, with the resistors R1, R2 which are connected inseries with one another and form a parallel branch in the basebandsignal path, being directly connected to one another by a furtherconnection in each case. A first respective switch S1, S2 is connectedin parallel with the first resistors R1, R2.

The arrangement shown in FIG. 1 is a high-pass filter with a switchablecut-off frequency, with the cut-off frequencies being formed from thecomponent values of the capacitances C1, C2, of the resistors R1, R2 andfrom parasitic elements.

The switches S1, S2 are preferably switched at the same time and areconnected to a control circuit.

The circuit shown in FIG. 1 is preferably suitable for use in receiverarrangements which are based on the direct conversion principle, that isto say for homodyne reception architectures.

The circuit shown in FIG. 1 in this case allows continuous receptionoperation even when the gain factor of an amplifier which is connectedupstream of the AC coupling is changed during normal reception operationfor matching the received field strength. For this purpose, a changeoveris made to a considerably higher cut-off frequency when the gain factoris changed, for example by closing the switches S1, S2, so that a suddenDC change which is caused by the switching of the gain decays with avery short time constant. The cut-off frequency of the high-pass filterthat is produced when the switches S1, S2 are open may in this case bechosen to be very low, for example around 2 KHz, so that the usefulsignal is transmitted virtually without any distortion. Overall, only aminimum amount of the information is lost from the useful signal when asudden change in gain occurs. Any transients which may occur whenswitching the gain remain negligibly small. An analog/digital converterwhich is connected downstream from a baseband chain in a receiver withAC coupling as shown in FIG. 1 can thus be driven at the optimumoperating point, so that the converter can be designed to have aparticularly small number of bits for digitization. Since sudden gainchanges can be carried out during normal reception, the described ACcoupling is suitable for code division multiple access methods, asenvisaged in the UMTS Standard.

FIG. 2 shows the time-domain profile of the voltage in microvoltsplotted against the time in microseconds for a signal A at the output ofthe filter arrangement shown in FIG. 1, when an amplifier to whose gaina change is made is arranged at its input. The filter arrangement shownin FIG. 1 in this case has a low cut-off frequency of 2 KHz, and a highcut-off frequency of >1 MHz. The DC offset is in this case about 5 mV,and the gain changes suddenly from 18 dB to 12 dB, with the −6 dB suddenchange in the upstream amplifier being made with a soft transition. Ascan be seen, the stabilization time for the filter arrangement has beenreduced to <1 microsecond, and the transient relating to the switchingtime t=1 μs is restricted to 0.6 mV. Without any switching of thecut-off frequency for a filter with a fixed cut-off frequency of 4 KHz,a DC offset of 5 mV and a sudden voltage gain change likewise from 18 to12 dB, the high-pass filter would stabilize only very slowly, with thestabilization time being more than 100 μs. Furthermore, a transient of20 mV would occur which would still be amplified many times, overall upto 50 dB, by downstream amplifier stages.

FIG. 3 shows the step-function response B of a receiver arrangement witha low-noise variable-gain baseband amplifier and the downstream filterarrangement as shown in FIG. 1, measured at the output of a programmableamplifier which is connected downstream of a channel filter which is inturn connected to the output of the filter arrangement. This shows thatthe stabilization time of this overall baseband chain when a change ismade to the gain factor of the baseband amplifier at the input of thebaseband chain is restricted to about 3 microseconds and that themaximum transient that occurs in the process and which does not exceed 4mV can, as an approximation, be ignored.

FIG. 4 shows the signal relationships on the basis of a signal profile Cin the situation where the actual useful signal is not considerably lessthan the sudden DC change that is caused by switching the gain. Duringthe changeover mode, that is to say when the switches S1, S2 are closedas shown in FIG. 1 and in which the high-pass filter has the highcut-off frequency, the voltage across the capacitors C1, C2 follows theinput signal. When switching back to normal operation by opening theswitches S1, S2, a sampling effect occurs in this case, that is to saythe instantaneous signal across the capacitors C1, C2 is sampled anddecays with the long time constant, that is now selected once again, fornormal operation, that is to say with the cut-off frequency of 2 KHz.Such relationships can occur in particular at the start of the basebandchain where, on the one hand, AC coupling is required, since thefrequency converters for conversion of the radio frequency to basebandhave manufacturing-dependent DC offsets while, on the other hand,adjacent channels still exist without being filtered, since the ACcoupling is carried out directly at the output of the mixers, and thuseven before the channel filter in baseband. FIG. 4 describes this signalvoltage as a function of the time at the output of the baseband chain,that is to say downstream from the programmable amplifier and upstreamof the analog/digital converters.

FIG. 5 shows a filter arrangement which has been developed from thatshown in FIG. 1 and which, in addition to the high-pass filter R1, R2,C1, C2 with a switchable cut-off frequency, has a low-pass filter R3,C1; R4, C2 which can be connected and is connected upstream of thishigh-pass filter. In this case, a series resistor is connected upstreamof each of the series capacitors C1, C2 in the balanced signal path, andis annotated R3, R4. A second switch S3, S4 is connected in parallelwith these series resistors R3, R4, respectively. The switch positionsof the switches S1 to S4 are shown during normal operation in thepresent FIG. 5.

During a changeover mode, firstly, as already explained with referenceto FIG. 1, the higher cut-off frequency is activated for the high-passfilter by closing the switches S1, S2, while the low-pass filter formedby R3 with C1 and R4 with C2 is additionally activated by opening theswitches S3, S4. The precise switching sequence must in this case becarried out in order to avoid the sampling effect with the controlcircuit which drives the switches S1 to S4, such that the switches, S3,S4 are opened first of all, after which the switches S1, S2 are closed,for switching from normal operation to the changeover mode. Whenreturning from the changeover mode to normal operation, the switches S1,S2 are opened again first of all, and the switch S3 and the switch S4are closed only after this has occured, in order to avoid the samplingeffect. The charge on the capacitors C1, C2 is accordingly changedduring the changeover mode with the time constants formed from theproduct of the resistance and capacitance, R3*C1 or R4*C2. This ensuresthat no sampling effect occurs at the output of the filter arrangement,even if the useful signal is not significantly less than the sudden DCchange caused by switching the gain.

FIG. 6 shows the signal profile D at the output of a baseband chain whenusing a circuit as shown in FIG. 5. This shows that the sampling effectno longer occurs. The stabilization processes that can still be seenduring the changeover mode are caused by the channel selection filterand/or the baseband filter. These can be reduced considerably, forexample, by reducing the gain after the channel selection, that is tosay by means of the conventional programmable amplifier there, or can beblanked out by insertion of a further AC coupling in the signalprocessing chain.

FIG. 7 uses an example of a simplified block diagram to shown a homodynereceiver arrangement with AC coupling which is, for example, in the formof a filter arrangement as shown in FIG. 1 or as shown in FIG. 5. Thereceiver is in this case in the form of an IQ receiver for processing ofcomplex-value signals and, in addition, for carrying the in-phase andquadrature components in each case as balanced signals.

In detail, an IQ mixer is provided for frequency conversion 1, and canbe supplied with a received radio-frequency signal, in each case in theform of a balanced signal, at in each case one signal input. The mixers1 also each have a local oscillator input, to which a local oscillatorsignal is supplied on the one hand unchanged and on the other hand witha phase shift of 90°. A baseband signal is produced on the output sideof the frequency converters 1, and is likewise in the form of balancedsignal. A low-noise variable gain baseband amplifier 2 is connected tothe outputs of each of the frequency converters 1 in the in-phase andquadrature paths of the receiver. A high-pass filter with a variablecut-off frequency for AC coupling is connected on the output side to thelow-noise baseband amplifier 2. A baseband filter 4 is coupled to eachof the outputs of the high-pass filters 3 for channel selection, andeach output of said baseband filter is in turn connected to aprogrammable amplifier or PGC, Programmable Gain Control 5. On theoutput side of the programmable amplifiers, that is say at the output ofthe baseband signal processing chain that has been explained, a filteredand amplified baseband signal is produced as a complex-value signal,broken down into an in-phase component I and a quadrature component Q,with the I and Q components each being in the form of balanced signals.

The low-noise baseband amplifiers 2, the filter arrangements 3 and theprogrammable amplifiers 5 each have a control input which is connectedto a control circuit 6. The signal for controlling the gain of thelow-noise baseband buffer 2 is annotated E, the signal for varying thedesired cut-off frequency of the high-pass filter 3 is annotated F, andthe signal for varying the gain of the programmable amplifier 5, inparticular the minimum gain which can be set for it, is annotated G.

The method of operation of the circuit illustrated in FIG. 7, which issuitable for receiving signals coded using the code division multipleaccess method and which accordingly can be used for UMTS receivers, isevident from the signal profiles of the control signals E, F, G shown inFIG. 8.

FIG. 8 shows the signal profile of the control signal E for thelow-noise baseband amplifier 2 which, at the time, T0, is reducing thegain by 6 dB owing to a change in the reception conditions, for example,a greater received field strength. The −6 dB step in this case has asoft gain transition. At the same time as the change to the gain, thefilter arrangement 3 is switched to a higher cut-off frequency for avariable time interval ΔT, that is to say the high-pass filters in thefilter arrangements 3 change from normal operation to the changeovermode, starting at the time T₀ and for the time period ΔT. If the filterarrangement as shown in FIG. 3 is additionally designed with a low-passfilter characteristic as shown in FIG. 5, then this low-pass filter isconnected upstream of the high-pass filter for the time period ΔT. Theprogrammable amplifier 5 at the end of the baseband signal processingchain is likewise switched by the control signal G to the minimum gainwhich can be set for it for the time period ΔT and starting at the timeT₀ in order for it not to amplify any further a switching transient,which is invariably unavoidable even if it is very small.

The combination of the accelerated AC coupling with the higher cut-offfrequency, the soft switching of the gain as shown by the signal E andthe switching off of the downstream amplifier stage 5 for thestabilization time ΔT which is now short of the accelerated high-passfilter as shown by the control signal G results in the describedadvantages, that is to say a low cut-off frequency during normaloperation of the high-pass filter 3, a short stabilization time after achange to the gain, negligibly small transients relating to theswitching times for the gain of the amplifier 2, an optimum drivecapability for downstream analog/digital converters with the usefulsignal, and thus an implementation with a small number of bits as wellas the capability to switch the gain of the amplifier 2 during normalreception.

Instead of the described process of switching the gain of the downstreamamplifiers 5 back in accordance with the control signal G, further ACcouplings may also be inserted into the baseband chain in alternativeimplementations.

LIST OF REFERENCE SYMBOLS

-   1 Frequency converter-   2 Low-noise baseband amplifier-   3 AC coupling-   4 Channel filter-   5 PGC-   6 Control circuit-   A Transient-   B Step-function response-   C Sampling effect-   D Signal profile-   E Control signal-   F Control signal-   G Control signal-   C1 Capacitor-   C2 Capacitor-   R1 Resistor-   R2 Resistor-   R3 Resistor-   R4 Resistor-   S1 Switch-   S2 Switch-   S3 Switch-   S4 Switch-   T₀ Start of the changeover mode-   ΔT Changeover mode interval

1. A receiver arrangement with AC coupling, comprising: an input of thereceiver arrangement configured to receive a radio-frequency signal; afrequency converter which is coupled to the input of the receiverarrangement and produces a baseband signal at its output; a basebandamplifier with variable gain, which is connected to the output of thefrequency converter and has a control input for varying the gain; afilter arrangement for AC coupling, with an input which is connected toan output of the baseband amplifier and with a high-pass filter forfiltering the baseband signal, with a cut-off frequency which can beswitched between at least two values, with the lower of the at least twocut-off frequencies being used during normal operation and with thehigher of the at least two cut-off frequencies being used in achangeover mode, and with a control input for varying the cut-offfrequency; and a control circuit for activation of the higher cut-offfrequency during the changeover mode on the basis of a readjustment ofthe gain of the baseband amplifier, with an output which is connected tothe control inputs of the baseband amplifier and of the filterarrangement, wherein the control circuit is configured to activate thehigher cut-off frequency in the high-pass filter of the filterarrangement during a variable time interval, which starts at the time ofthe change to the gain of the baseband amplifier on the basis of thereadjustment of the gain.
 2. The receiver arrangement as claimed inclaim 1, wherein the filter arrangement comprises a selectivelyactivatable low-pass filter coupled to the control circuit in order toactivate the low-pass filter effect during the changeover mode.
 3. Thereceiver arrangement as claimed in claim 2, including a programmableamplifier coupled to the output of the filter arrangement and connectedto the control circuit, in order to set the lowest gain which can be setfor the programmable amplifier during the changeover mode.
 4. Thereceiver arrangement as claimed in claim 1, including a programmableamplifier coupled to the output of the filter arrangement and connectedto the control circuit, in order to set the lowest gain which can be setfor the programmable amplifier during the changeover mode.
 5. Thereceiver arrangement as claimed in claim 1, wherein the filterarrangement comprises a selectively activatable low-pass filter coupledto the control circuit in order to activate the low-pass filter effectduring the changeover mode.
 6. The receiver arrangement as claimed inclaim 5, including a programmable amplifier coupled to the output of thefilter arrangement and connected to the control circuit, in order to setthe lowest gain which can be set for the programmable amplifier duringthe changeover mode.
 7. The receiver arrangement as claimed in claim 1,including a programmable amplifier coupled to the output of the filterarrangement and connected to the control circuit, in order to set thelowest gain which can be set for the programmable amplifier during thechangeover mode.
 8. The receiver arrangement as claimed in claim 7,including a baseband filter for channel selection connected between thefilter arrangement and the programmable amplifier.
 9. The receiverarrangement as claimed in claim 1, wherein the receiver arrangement isdesigned for processing balanced signals.
 10. The receiver arrangementas claimed in claim 1, wherein the high-pass filter in the filterarrangement comprises a balanced signal path having therein a parallelbranch that includes a resistor and a parallel-connected first switch,the balanced signal path including series branches connected upstream ofthe parallel branch, each said series branch including a capacitance.11. The receiver arrangement as claimed in claim 10, including abaseband filter for channel selection connected between the filterarrangement and the programmable amplifier.
 12. The receiver arrangementas claimed in claim 10, including a programmable amplifier coupled tothe output of the filter arrangement and connected to the controlcircuit, in order to set the lowest gain which can be set for theprogrammable amplifier during the changeover mode.
 13. The receiverarrangement as claimed in claim 10, wherein the filter arrangementcomprises a selectively activatable low-pass filter coupled to thecontrol circuit in order to activate the low-pass filter effect duringthe changeover mode.
 14. The receiver arrangement as claimed in claim13, including a programmable amplifier coupled to the output of thefilter arrangement and connected to the control circuit, in order to setthe lowest gain which can be set for the programmable amplifier duringthe changeover mode.
 15. The receiver arrangement as claimed in claim10, including a programmable amplifier coupled to the output of thefilter arrangement and connected to the control circuit, in order to setthe lowest gain which can be set for the programmable amplifier duringthe changeover mode.
 16. The receiver arrangement as claimed in claim10, wherein the filter arrangement comprises a selectively activatablelow-pass filter coupled to the control circuit in order to activate thelow-pass filter effect during the changeover mode.
 17. The receiverarrangement as claimed in claim 16, including a programmable amplifiercoupled to the output of the filter arrangement and connected to thecontrol circuit, in order to set the lowest gain which can be set forthe programmable amplifier during the changeover mode.
 18. The receiverarrangement as claimed in claim 10, wherein the series branches of thebalanced signal path in the filter arrangement each comprise a parallelcircuit formed by a resistor and a second switch in order to form withthe associated capacitance a low-pass filter, and wherein the parallelcircuits are connected upstream of the associated capacitances.
 19. Thereceiver arrangement as claimed in claim 18, wherein the parallel branchincludes a further resistor and parallel-connected first switch,connected in series with the first-mentioned resistor andparallel-connected first switch, and wherein the control circuit isoperable such that in order to activate the changeover mode, the secondswitches are first of all opened and the first switches are then closed,and such that, in order to return to the normal mode, the first switchesare first of all opened, and the second switches are then closed. 20.The receiver arrangement as claimed in claim 1, wherein the highercut-off frequency is greater than or equal to 1 Megahertz.